1. Field of the Invention
The present invention relates to an image formation apparatus like an electrophotographic copying apparatus for example. More particularly, the invention relates to an image formation apparatus which allows the optical scan system to perform return movement to the initial position with unlimited speed, i.e., with full power.
2. Description of the Prior Art
Any conventional image formation apparatus like an electrophotographic copying apparatus or an image scanner generates reciprocation of the optical scan system movably installed below a transparent platen in order to read the content of a document before transmitting the scanned optical image data either to a static latent image forming means or to a photoelectric conversion means via an optical means.
Taking an electrophotographic copying apparatus for example, the optical scan system consists of a light-exposure lamp for exposing the document to light, a number of reflection mirrors, and a lens, where the optical scan system reciprocates itself between a specific distance corresponding to the size of the document. When the copying apparatus remains inoperative, the optical scan system is at the original position which is called "home position". As soon as the copying operation is activated, the optical scan system starts to move itself, and simultaneously, the optical scan system reads the content of the document on the platen by exposing it to the scan light, and finally, the optical scan system returns to the home position.
In order to accelerate the speed of the copying operation, it is essential for the optical scan system to accelerate its return movement. To achieve this, it is quite important for the copying apparatus to prevent the optical scan system from colliding itself with the wall on the part of the home position. Based on this reason, a variety of devices are introduced to control the speed of the motor driving the optical scan system.
FIG. 1 graphically presents performance of a conventional system for controling the return movement of the optical scan system, in which solid line designates the movement under normal condition. The return movement of the optical scan system is controlled by detecting the actual position of the optical scan system by jointly operating a brake sensor BS and a home position sensor HPS.
First, the optical scan system starts up its return movement at time t0 in the direction of the home position at a very fast speed without applying any speed control means. Meanwhile the speed of the rotation of the optical-system drive motor increases relative to the passage of time. When the optical scan system arrives at the position of the brake sensor at time t1, the optical-system drive motor is driven in the inverse direction for a predetermined period of time to activate a braking force. As soon as the braking period is terminated at time t2, a phase-locked loop (PLL) mechanism is activated to control the speed of the rotation of the optical-system driver motor so that the optical scan system can move by itself at a specific speed. When the optical scan system arrives at the home-position sensor, the rotation of the optical-system drive motor is turned OFF at time t4 to complete the return movement of the optical scan system. After the time t4 is past, the optical scan system again starts to move itself in the forward direction to scan the content of the following document.
In order to accelerate the speed of the reciprocation of the optical scan system, it is essential for the image formation apparatus controlling the optical scan system to minimize the braking period by reversing the rotation of the drive motor before returning the optical scan system to the home position at a very fast speed. On the other hand, if the braking period were too short, the optical scan system cannot decelerate itself to the predetermined speed limit to avoid hitting against the wall of the home position. To prevent this, normally, any conventional image formation apparatus provides a certain braking period by reversing the rotation of the drive motor by providing the minimum period for preventing the optical scan system from hitting against the wall of the home position.
For example, when a copying apparatus continuously performs copying operations of documents, heat is internally generated from the light-exposure lamp to gradually raise temperature inside of the copying apparatus. As a result, coil resistance of the optical-system driving motor rises, whereas control current flowing through the coil decreases.
When full power is exerted to execute the initializing movement of the optical scan system at an unlimited speed in the conventional copying apparatus mentioned above, because of the decreased control current, immediately before reaching the time t1 at which braking force is applied by reverse rotation of the drive motor, the speed of the rotation of the drive motor slightly lowers as shown in FIG. 1 with a broken line. Nevertheless, the braking force applied in the inverse direction at the time t1 is subject to attenuation. In other words, the effect of attenuation dominates over the braking effect. In consequence, the optical scan system accelerates its returning speed at the time t2 at which the phase-locked loop control is activated.
If the braking duration were not provided with enough allowance against occurrence of collision with the wall of the home position, as mentioned above, the rise of temperature in the drive motor, and the moving speed of the optical scan system in the direction of the home position will gradually increase, thus causing collision to occur. If collision occurs, unwanted noise increases, and the durability of components decreases damage.
Provision of enough braking period effectively prevents the optical scan system from hitting against the wall of the home position. On the other hand, this in turn retards the operating speed of the optical scan system, thus eventually lowering the productive efficiency of the copying operation.